INTERNAL_FUNCTION_THROTTLED

The INTERNAL_FUNCTION_THROTTLED error occurs when the limit of concurrent executions for your Vercel Functions is exhausted (e.g., due to a sudden request spike, backend API slowness/outage, etc). For more information see What should I do if I receive a 503 error on Vercel?.

Although this is a rare scenario, this error can also occur when Vercel's infrastructure encounters an abnormal system load and tries to mitigate the impact autonomously.

To troubleshoot this error, follow these steps:

1. Check application logs: Review the application logs to identify any specific errors related to the Vercel Function being invoked. For example, your function might be waiting for a slow backend API without a reasonable timeout. These information can be found at the host URL under the /_logs path, as well as the Observability tab in the Vercel dashboard.
2. Handle request spikes: If you're experiencing a sudden spike in requests, consider using the Vercel Firewall to block unwanted traffic, or enabling Rate Limiting to limit the number of requests per second.
3. Optimize your function: Review your function code to ensure it's optimized for performance. For example, you can use Vercel Edge Caching to cache responses and reduce the number of invocations. You can also adjust the concurrency settings to handle more requests concurrently on a single function instance.